The objective of the project is to gain insight into the mechanism of exocytosis in exocrine secretory cells through studies which probe the specific interactions of isolated parotid secretion granules with other appropriate cellular elements, e.g., plasma membranes, other secretion granules, and small molecules like Ca2 ions and ATP. A Ca2 ion-specific phospholipase associated with rabbit parotid granules has been purified and characterized using unilamellar phospholipid vesicles as substrate. The enzyme is presently being tested for its possible role in the Ca2 ion-stimulated hydrolysis of phospholipids of the granule membrane and in Ca2 ion-stimulated secretion granule lysis. Secretion granules, granule membranes and plasma membranes containing distinct apical domains have been obtained as cellular fractions from rat parotid gland. These fractions have been extensively characterized with regard to their contamination by other cellular organelles and their content of putative membrane markers; 5'-nucleotidase, alkaline phosphatase, alkaline phosphodiesterase, gamma-glutamyl transpeptidase, leucine aminopeptidase, and potassium-stimulated paranitrophenyl phosphatase. The fractions appear appropriate for use in attempts to obtain an in vitro fusion model for exocytosis. Mast cells and collagenase-dissociated parotid acini are being characterized as intact secretory systems in which in vivo correlates will be sought for each of the in vitro findings.